Stuffed in cakes, sold in gilt gift boxes, and a feature of many expensive menus – giant sea snails, better known as abalone, are a centerpiece of haute cuisine in Hong Kong and a highly lucrative business for those who know where to find them.

The catch? The product isn’t always legal.

High demand for abalone is fueling overharvesting around the world, especially in South Africa, one of Hong Kong’s largest suppliers of the chewy, fishy delicacy. Another illegal batch was caught this week when Hong Kong officials returned a massive 2.6-ton shipment of dried abalone to South Africa, where they’d been poached over a year ago before being smuggled to Hong Kong last March.

Hong Kong is the epicenter of the world’s abalone trade, importing nearly half the world’s production. Dried abalone are sold throughout the island’s western reaches, piled in barrels like crusty, deflated balloons. Apart from what’s consumed locally, much of the abalone winds up across the border in mainland China or shipped elsewhere in the region. The product is seen as an aphrodisiac, as well as a comestible that can benefit the liver, eyes and heart.

Rising Asian demand has made the global abalone population especially vulnerable to overfishing, says Allen To of the World Wildlife Foundation. Essentially oversized sea slugs, abalone are easily poached: All it takes is plucking them off the rocks to which they cling in shallow coastal waters.

Hong Kong’s local abalone population was long ago over devastated by overfishing. “When you go to the markets here, none of it is local anymore,” says Mr. To. South Africa, where limits on how many abalone can be harvested are regularly violated, has likewise been hard hit by overfishing, he says.

According to government data, between 2007 and 2009 alone, South Africa confiscated 107 tons of illegally harvested abalone, just a fraction of what’s actually poached in the region. Many poachers are local, WWF notes; however, last year three Chinese nationals were imprisoned in South Africa for sentences of 30 months and up for “abalone-related crimes.”

For consumers looking to indulge their cravings, Mr. To notes, a smarter, more sustainable source of abalone is actually mainland China. In northeastern Dalian, he says, abalone is harvested using “full-cycle culture,” which focuses on farming stock produced in captivity.

According to South Africa’s fisheries department, the dried abalone will be auctioned, with proceeds going to protect the country’s marine resources. Though the abalone have already arrived in South Africa, they’d be better off being sold back in Hong Kong, where the price of a single kilogram abalone of abalone can be as high as 30,000 Hong Kong dollars (or US$3,900).